Thanks very much for your help.
When I looked at the Update section of the UI later, the Check for Updates was no longer greyed out, so I was able to update manually.
If the virus definitions are out of date for any reason, would it not be helpful if there was a pop-up warning? As a new user of Premium Security, I am looking at the various sections of the UI quite often to familiarise myself with the program, but I imagine that confirmed users do not routinely go to the Update section, and would therefore not know that there was a problem.
Apart from this issue, i have been very happy with my trial of Premium Security.